Electric control system



July 27, 1937. s. c. HUBBLE 088,295

ELECTRIC CONTROL SYSTEM Filed Oct. 24, 1954 3 Sheets-Sheet 1 Jul 27,1937.

G. c. HUBBLE 2,088,295

I ELECTRIC CONTROL SYSTEM Filed Oct. 24, 1954 s Sheets-Sheet 2 gy mmFiled Oct. 24, 1934 5 Sheets-Sheet 3 Patented July 27, 1937 UNITEDSTATES PATENT OFFICE 17 Claims.

This invention relates broadly to sensitive electrical circuits adaptedto respond to the approach or contact of a foreign body to initiate achange in current flow in the circuit. It may be employed for variouspurposes but for purposes of illustration it is described herein asapplied to a safety system for detecting the presence of burglars orother intruders in a protected space.

A broad object of the invention is to provide a sensitive electricalcircuit capable of responding to the approach and/or contact of aforeign body, which circuit is stable and reliable in operation and iscapable of handling control or alarm conductors of substantial lengthand capacity.

Various other more specific objects and-features of the invention willbecome apparent from the detailed description with reference to thedrawings which follow.

This is a continuation in part of my application Serial No. 691,090,filed September 26, 1933 on Electronic discharge systems, which has nowbecome abandoned.

In accordance with my invention, I employ as the main element of mysystem a glow tube having a cathode, anode and control grid, in anatmosphere of a rare gas, such as neon or argon. It is well known thatsuch tubes have the characteristic of being substantially non-conductiveat anode-cathode potentials below a definite value but suddenly becomeconductive if the anodecathode potential is increasedabove that value orif the potential of the grid is varied beyond definite limits. Becauseof this characteristic, such tubes have been employed by others indelicate detection circuits so arranged that the mere presence of aforeign body adjacent a conductor connected to the grid of the tubewould alter the potential of the grid sufliciently to initiate adischarge in the tube, the resultant current flow being employed tocontrol a switch in an alarm circuit, alighting circuit, or the like.

All these prior circuits of which I have knowledge have definitelimitations in use. I have found in general that they are relativelyinsensitive and unreliable and lack sensitivity when employed withdetecting conductors of any appreciable length.

The present invention involves novel circuits and apparatus incombination with a grid glow tube whereby the stability, reliability andsensitivity are greatly improved. My circuit and apparatus will now bedescribed in detail with reference to the accompanying drawings, inwhich Fig. 1 is a wiring diagram of a double unit auto-matic alarmsystem embodying my inven tion? Fig. 2 is an elevation view of thedouble unit with the enclosing wall and cover of the containcr shown insection;

Fig. 3 is a plan view of the double unit disclosed in Figs. 1 and 2;

Fig. 4 is a transverse sectional view of the unit taken in the plane'IVIV of Fig. 2;

Fig. 5 is a section-a1 view of a transformer employed in the unit; and

Fig. 6 is a schematic diagram of the detecting circuit employed in eachof the units disclosed in Fig. 1.

Referring first to Figs. 1, 2, 3 and 4, my automatic alarm systemapparatus comprises a metal container l divided into two compartments Aand B, respectively, by a metal partition 2. A plug 3 is provided in oneside wall of the container l for making connection to an external sourceof alternating current, which source is preferably the 110-voltcommercial lighting circuit.

Each compartment A and B contains apparatus which is substantiallyidentical with th-at'in the other compartment. This apparatus will bedescribed with reference to the contents of com partment A.Corresponding elements of the apparatus in compartment B bear the samereference numeral with the prime mark added.

Thus referring to Fig. 1, compartment A contains a special transformer4, three relays 5, 6

and 1, respectively, four variable condensers 8,

9, l0 and H, respectively, a .variable inductive resistance l2, 2. gridglow tube 13, a voltage control unit l4, and other minor elements whichwill be described in connection with the circuit description.

As disclosed in Figs. 2, 3 and 4, the metal container l is in the formof a staunch steel box containing the previously listed parts which areheld in definite spaced relation within the box.

Extending from each of the compartments A and B of the metal container II preferably provide a flexible conduit 16a leading to a metallic alarmor gong box I6, which conduit carries \three conductors 59, 60 and 82,respectively.

Each alarm box 16 contains'a battery I! which may consist of fourstandard dry cells, an electric bell l8 and a siren l9.

Referring now to the apparatus within compartment A, it will be observedthat transformer 4 comprises a primary winding 2| connected to theterminals of the plug 3. Thus the upper end of winding 2| is connectedthrough a conductor 3| and one of the windings of the reactance elementIE to one terminal of the reversing switch Hit). The lower end ofwinding 2! is connected through a conductor 285a and the other windingof reactance element M to another terminal of the reversing switch ltd.One of the remaining terminals of switch llld is connected through aswitch 30 to a conductor 29 nected through conductor 28 to the otherterminal of the plug 3. Line potential connected to plug 3 willtherefore be appliedto the primary winding 2| when the switch 38 isclosed and} by manipulating the reversing switch I88, the direction ofthe current in the winding 2| may be reversed. The purpose of providingthe reversing switch I 88 will be explained later.

The secondary winding 22 of transformer 4 contains a great many moreturns than the primary winding 2| and is provided with numerous tapsthereon, these taps being preferably so spaced along the winding as toprovide a diiference of potential of substantially 500 volts betweeneach adjacent pair of taps. Several of the taps on the upper end of thewinding are shown connected to the contacts 81, 88, 88 and 88 associatedwith a pair of switch blades 12 and 88, respectively. The switch blade88 is connected through a lead 33 and the variable inductive resistanceelement l2 to the cathode 31 of the glow tube l3. The other switch blade12 is connected through a conductor 8|, winding of relay 5, andconductor 48 to the anode 48 of tube I3. The tube I3 is preferably aWestinghouse grid glow tube, type KU-618, in the actual construction oi!which the cathode 31 comprises a cylinder with the anode 48 constitutinga point projecting from a glass tube positioned substantially in themiddle oi the cylindrical cathode. The grid 28 consists of a pointprotruding from a glass tube immediately adjacent the anode 48. Theelectrodes, of course, are positioned in a sealed bulb containing a raregas at suitable pressure.

The grid 28 is connected through a conductor 4i and a high resistance42, having a resistance of the order of a million ohms, to a detectingconductor 43 which is extended through an insulating bushing in themetal case I to the exterior where it may be connected to an object tobe protected, such as a cash register 44.

The variable condensers 8, 8, I8 and II each have one terminal connectedto the conductor M and have their other terminals connected to taps 83,84, and 85 adapted to be successively connected by a movable contact 38,which is connected through a conductor 38 to the cathode 31 of tube I3.By shifting the movable blade 38, the condensers 8, 8, l8 and II may besuccessively connected into the circuit.

Condensers 8, 8 and I8 are manually variable, being connected to controlknobs on the exterior of the case. The condenser II is adapted to beautomatically varied by a solenoid Ila con-- nected directly to theconductors 28 and 28.

The anode 48 of tube I3 is connected through a high resistance elementI8 I, having a resistance of the order of a million ohms, to a conductorI82 which is extended through an insulating bushing in the metal case Ito the exterior and may, like conductor 43, be connected to an object tobe protected such as a safe I83. A switch I84 is provided within thecase I for connecting,

the conductor I82 to the metal case itself when desired.

When the circuit described is energized by connecting a -voltalternating current supply to the plug 3, the system can be adjusted toa state of equilibrium, in which substantially no current flows throughthe tube I3, by adjusting one or more of the condensers 8, 8 and I8.Thereafter, assuming no change takes place in the condition of thedetecting conductors 43 and I82 extending from the case, equilibriumwill be maintained despite variations in the potential across conductors28 and 28 by the action of the solenoid IIa which varies the capacity ofthe variable-condenser II to compensate for voltage variations in thepower source.

However, the state of equilibrium may be terminated by a change in theelectrical condition of either conductor 43 or conductor I82 extendingfrom the metal case. It is found that contact of almost any externalobject with the conductor I82 or an object connected thereto (such asthe safe I83) initiates a discharge between the cathode and anode withinthe tube l3. The conductor 43 is even more sensitive. Thus even theapproach, without actual contact, of a foreign object to the conductor43 or an object connected thereto is suflicient to initiate a dischargein the tube I3.

The initiation of a discharge in tube l3 causes current to flow throughthe relay 5, closing the armature 41 of that relay on its front contact48, which completes a circuit between conductor 68 and conductor 58.This applies current from battery II to the bell I8 and the siren I8,thereby producing an alarm signal. This action continues so long as theforeign object is in contact with conductor I82 or adjacent conductor43. However, if the object is removed the discharge in tube I3 ceasesand relay 5 releases, stopping the bell l8 and siren I8.

In some instances, however, it is desired to produce an alarm signalcontinuously upon initiation thereof. .It is for this purpose that relay8 is provided. It will be observed that relay 8 has an energizingwinding 48 adapted to be connected through a switch 52 with conductor.

82, which is connected to one side of the battery I1 in the alarm boxIt. The other side of battery I! is connected to conductor 58, which inturn is connected to the other side of winding 48. Therefore, closure ofswitch 82 completes a circuit from one side of the battery I! throughthe conductor 82 up to the contact 48 of relay 5. Immediately uponactuation of relay 5 a circuit is completed through winding 48 andcontact 48 and armature 41' of relay 5 to the conductor 88 which isconnected to the other side of battery I! and the relay 48 pulls up.When it pulls, this relay closes a pair of holding contacts 5i and 52connected in shunt to the contact 48 and armature 41 so that the relay48 remains locked up until the switch 52 is opened. The closure ofcontacts 5I and 52 of course completes the energizing circuit to thebell l8 and the siren I8 in the same manner as did relay 5.

Relay G'is also provided with a pair of contacts 54 and 55 and a switchblade 53 adapted to close contacts 54 and 55 when said relay 8 isenergized. This completes a circuit from conductor 28 to contact 55,switch member-53, contact 54, and conductor 84 to one terminal of areceptacle 83 positioned in the exterior wall of the case I. The otherterminal of this receptacle is permanent-ly connected to the conductor28. By connecting an external circuit to the receptacle 83 by a suitableplug, the circuit may be controlled in response to the actuation of thetube I3. This external circuit may consist of the lighting circuit ofthe building so that in the case of an alarm the building is floodedwith light.

Inan alarm system dependent upon the continuity of current from acommercial power systern, it is-desirable to provide means for giving analarm in case of failure of the power supply. To this end, relay I isprovided. This is an alternating current relay bridged directly acrossthe conductors 28 and 29 and is provided with an armature 51 and a backcontact 58, which are connected to the conductors 60 and 59,respectively, so that release of relay 1 actuates the bell l8 and thesiren I9. Ordinarily the armature 57 is held clear of the contact 58' bycurrent from the source conductors 28 and 29.

r The layout of the apparatus in compartment A as a unit and itsoperation have been briefly reviewed. Certain elements of the apparatuswill now be described in greater detail.

The transformer 4 constitutes a very important element of my apparatusand the success of the system depends in large measure on thecharacteristics of this transformer. As previously indicated, theprimary winding 2| is so proportioned as to normally substantiallycompletely saturate the core. proportioning the primary winding as tosaturate the core, the stability of the system is greatly increased andthe magnitude of the secondary potential is affected to a much lesserdegree by variations in the line potential across conductors 28 and 29.Considerable experimentation was required to produce a transformerhaving the desired characteristics and for this reason the constructionof a suitable transformer will be described in detail with reference toFig. 5.

It will be observed that the transformer is provided with a core 28 of Hshape consisting of lamination's of. silicon steel. The externaldimensions of the core are approximately 3 3% inches andthe laminationsare built up to a depth or 35 thickness of about 1% inches. Bothwindings are placed on the central cross bar, the primary winding 2|consisting of 394 turns of No. 24 enameled wire. The secondary windingis placed on top of the primary and is Wound in sections, 40 which arepreferably separated from each other by layers of insulating material.The first or inner section 23a of the secondary winding may consist of880 turns of No. 37 enameled wire; the second section 231) consists of1760 turns; the third section 230 consists of 3520 turns; the fourthsection 23d consists of 5280 turns; the fifth section 23c consists of7040 turns; and the sixth section 23; consists of 8080 turns, all of No.37 enameled wire. It is found that with the Various sections of thesecondary proportioned as described, approximately 500 volts will bedeveloped between the ends of each section, which is the desired object.

In part because of the small number of turns in the primary winding 2|,this winding would draw excessive current from the line were itconnected directly thereto; Therefore, to prevent excessive current flowand to provide a means for regulating the current flow in the pri- 0mary winding, vI provide the variable reactance element |4 connected inseries with the winding- 2| and the supply conductors 28 and 29.Reactances of the general type which I prefer to use are fully describedin my United States Letters Patent No. 1,628,148; The essential featuresof this reactance are that it comprises two independent windingsfoneconnected in each side of the line and both windings closely adjacenteach other. Variation in reactance is preferably obtained by means of amovable core 34 r In Fig. 6 of the drawingsone of the conductors "beunderstood that in actual construction .both

conductors would be of the-same dimensions.

It is found that by so' The condensers are preferably proportioned aboutas follows: condenser 8 may be a variable 23-blade midget variablecondenser having a maximum capacity of 100 micro-microfarads; condenser9 may be a 35.-blade large variable condenser having a maximum capacityof 325 mlcromicrofarads; condenser I!) may be a small variable condenserhaving a capacity variation from .0001 to .0005 microfarads; and thecondenser H may have a capacity variation of .0003 to .001 microfarads.

By virtue of the fact that the lower end of the secondary winding 22 oftransformer 4 (with reference to Figs. 1 and 6) is closely adjacent tothe primary winding 2|, there is considerable capacity between primarywinding 2| and the lower endof winding 22. Furthermore, since primarywinding 2| is connected to a commercial power circuit, oneside of whichis invariably grounded, one end of primary winding 2| will besubstantially at ground potential and by virtue of the capacity betweenthe windings the lower end of the secondary winding 22 will bemaintained to a greater or less extent at ground potential. If desired,the potential of the lower end of winding 22 may be definitely fixed bygrounding it, as shown by the dotted connection 0 in Fig, 6. However, inactual practice I have not found it necessary to provide this groundconnection.

As previously stated, the secondary Winding is so proportioned thatsubstantially 500 volts are developed between adjacent taps thereon sothat regardless of which set of taps the conductors 8| and 33 areconnected to the same voltage will be impressed between the anode 20 andcathode 31 of the tube |3. However, the potential of the tube electrodesconsidered as a unit with respect to ground will depend upon which setof taps on winding 22 the conductors 33 and 8| are connected across.

This feature of maintaining the tubes as a whole, including thedetecting conductors 43 and I02, at a potential substantially aboveground potential, is an important novel feature of the invention and onethat contributes greatly to the superior performance of my system ascompared to prior known systems. As previously indicated, the potentialof the tube as a unit with respect to ground may be varied withoutvarying the potential difference impressed between the cathode and anodeby manipulating the switch to connect the switch blades 12 and 80 todifferent pairs of the taps 81,08, 89 and 90, since the transformer isdesigned to develop the same potential, approximately 500 volts, betweeneach adjacent pair of taps. With different external circuit conditionsof the conductors 43 and I02, different positions of the switch 80 maybe found to give best results, the best position in each case beingdetermined by experiment.

It is important to note that despite the fact that the conductors 43 andI02 are maintained substantially above ground potential, no injury canresult to a person touching them and no injury to the tube or otherapparatus can result from a ground on these conductors because of thehigh resistance elements 42 and IM connected in series with therespective conductors within the case these resistances limiting anycurrent flow in the conductors to an extremely small value.

I am unable to offer any definitely correct detailed explanation of thetheory of operation of mysystem because I have been unable to obtainfrom the literature relating to grid glowtubes any definitely correctdetailed explanation of what takes place within the tubes. I am inclinedto believe that there is normally (with voltage impressed across theelectrodes but no discharge) an accumulation of electrons on the grid 20(Fig. 6) which imparts to the grid a negative charge, the averagepotential of the grid with respect to the cathode and anode beingdependent upon the rate of accumulation of electrons thereon and therate of leakage therefrom through the conductor 43 and the objects andapparatus connected thereto. Alternating potentials are also impressedupon the grid through thecondenser 8 connecting'the grid to the cathodeand through the inherent capacity between the conductor 43 and ground.These alternating potentials on the grid probably control to a largeextent the rate of accumulation of electrons on the grid andhenceindirectly control the average potential of the grid. By suitableadjustment of the condenser B with respect to the capacity to ground andleakage conductance to ground of the con ductor 43, the averagepotential of the grid may be adjusted to a value only slightly differentfrom that required to produce a discharge in the tube. Thereafter aslight change in the capacity or conductance to ground of the conductor43 produced by the approach or contact of a foreign body producessufllcient change in the potential of the grid to initiate .a dischargein the tube. Likewise a change in the conductivity of the airsurrounding the conductor 43 or an object connected there to, such asmight be produced by the ionizing effect of a flame, is sufficient tostart the discharge. This feature renders the invention useful for firealarm purposes as well as burglar alarm.

A discharge may also be started in the tube, without varying thepotential of grid 20, by increasing the potential drop between thecathode 31 and anode 40 above predetermined limits.

40 This explains the action of the tube when the conductor I02 iscontacted by a body connected to or having appreciable capacity toground, as under these conditions the relatively high potentialdeveloped in the lower portion of the transformer secondary winding 22is applied through ground, the foreign body H5, and conductor I02 to theanode 40. It should be noted that the conductor I02 is much lesssensitive than the conductor 43 but has the advantage that it may beextended to much greater distances. Thus I have found that the conductorI02, if reasonably well insulated from ground, may be extended for mileswhereas the length of conductor 43 is limited to a few hundred feetunder most conditions.

It is to be understood that it is not necessary to use both conductors43 and I02 unless conditions make it desirable to do so. Either one maybe eliminated without effecting the operation of the other.

The conductor I02 may, if desired, be connected to the metal of theapparatus by closing switch I04 (Fig. 1), thereby sensitizing the caseso that if anyone attempts to tamper with the system and touches thecase, an alarm will be produced.

If it is desired to render the case of the instrument sensitive to theapproach without actual contact of a person, the case may be connectedto conductor 43 by closing switch I20 (Fig. 1).

The adjustable inductive resistance, I2 functions as a safety device tolimit the current flow through the tube I3. However, it is also useful"In balancing the system.

The variable reactance I4 in the primary circult of the transformer 4 isemployed to regulate the current in the primary winding M. This isnecessary, in part, because the transformer primary has so few turnsthat excessive current would flow therein if no regulating apparatuswere employed. The particular type of reactor described, in which thetwo windings are coextensive and alongside each other, has been found byexperiment to be particularly effective in the system-increasing itssensitivity-although I have not yet determined the explanation for thispeculiar efficacy.

Referring back to Fig. l, the apparatus in compartment B issubstantially identical with that in compartment A and may be employedindependently thereof. However, it is sometimes advantageous to employthem both in combination and there are two ways of doing this.

One method is to connect the two conductors 43 and 43 together and tothe same objects, as by closing switch 85, and so positioning thereversing switch I00 in compartment A as to cause the alternatingcurrent applied to the tubes I3 and I3, respectively, to be in phase.With this arrangement approach of a body to either conductor 43 or 43will initiate a discharge in one or both of tubes I3 and I3, dependenton whether or not either associated circuits are balanced with equalprecision. The important advantage of this connection, however, is thata break in either conductor 43 or 43' upsets the balance of the systemand initiates a discharge in one or. both tubes, thereby making itpractically impossible to open the broken circuit without giving analarm.

It is to be understood that the conductors I02 y and I02 may also beconnected together and to the same objects, the reversing switch I00being so positioned that the potentials applied to the anodes 40 and 40are in phase with each other. This connection may be effected by closingswitch I02a. The system will then respond to an open circuit conditionon either the conductors I02 or I02 as well as to contact of a foreignobject with either conductor.

The other method is to maintain the two conductors 43 and 43'electrically insulated from each other and position them on oppositesides of an area to be guarded and adjust the reversing switch I00 sothat the alternating current applied to the tubes I3 and I3 are oppositein phase. Thus in Fig. 1 the conductors 43 and 43 are shown extendedrespectively above and below a window 86 and the switch 85 is open.Under these conditions the conductors 43 and 43' have alternatingpotentials impressed thereon through their associate tubes, whichpotentials are of radically different instantaneous values so that aslight change in the capacity and/or conductance characteristicsproduced by the interposition of any body therebetween causes a changein the potential of one or both of the grids 20 or 20 sufllcient toinitiate a discharge in the associated tube, the superior sensitivity ofthe conductors 43 and 43' with respect to detection of a bodytherebetween, as compared to a body approaching only one of theconductors, being due to the fact that the alternating current potentialdifference between the twov conductors is substantially equal to the sumof the potentials. of the two conductors with respect to ground.

It is to be understood that although I have described what I nowconsider to be the preferred embodiment of the invention that it is notess ntial that the system be reproduced exactly as described. Variousmodifications will occur to those skilled in the art and may be employedwithout departing from the essential features of the invention, whichare set forth in broad terms in the appended claims.

I claim:

1. In a system for controlling an electric circuit in response to changeof a body with respect to a portion of said circuit; a source ofalternating current, a load device, an electric valve havinga pair ofelectrodes spaced apart from each other, said device conductingsubstantially no current below a predetermined critical potential andbecoming conductive above said critical potential, means connecting saidelectrodes and said load device in series with said source, means forimpressing a potential between ground and all parts of the seriescircuit including said source, said load device, and said electrodes, adetecting conductor connected to one of said electrodes and insulatedfrom ground, said source impressing a potential across said electrodesslightly less than said critical potential whereby contact between saiddetecting conductor and an external body applies a potential from saidlast named means i through ground and said external body to saiddetecting conductor, thereby varying the potential of said conductorsuflicient to render said valve conductive.

2. A system as described in claim 1 in which the said means forimpressing a potential between said electrodes and ground-is also asource of alternating current.

3. A system as described in claim 1 in which said source of current andsaid means for impressing a potential between said electrodes and groundare constituted by a transformer having a secondary winding with tapsthereon, one end of said winding being substantially at ground potentialand a tapped portion of said winding remote from said one endconstituting said source connected to said electrodes.

4. In a system for controlling an electric circuit in response to changeof a body with respect to a portion of said circuit; a load device, anelectric valve having a pair of electrodes spaced apart from each other,said valve conducting substantially no current below a predeterminedcritical potential and becoming conductive above said criticalpotential, a transformer having a core of paramagnetic material, aprimary winding and a secondary winding, means for connecting saidprimary winding to a source of alternating current, means formaintaining one end of said secondary winding substantially at groundpotential, said secondary winding comprising a tapped portion remotefrom said grounded end connected in series with said load device and theelectrodes of said electric valve, said tapped portion of said secondarywinding delivering a potential to said electrodes slightly less thansaid critical potential, a detecting conductor connected to one of saidelectrodes and insulated from ground whereby contact of an object withsaid conductor increases the potential applied to said electrodes abovesaid critical value and initiates a discharge through said valve, thesecondary winding of said transformer having a plurality of taps there-70 on and the number of turns'between the taps being so proportionedthat the potential delivered between any adjacent two of said severaltaps is slightly below said critical potential whereby said electrodesmay be connected across different taps to apply the same voltage acrosssaid electrodes While applying different potentials between theelectrodes and ground.

5. In a system for controlling an electric circuit in response to changeof a body with respect to some portion of said circuit; a source of a1-ternating current, a load device, an electric valve having a cathode, ananode and a control electrode, means connecting said cathode, anode andload device in series with said source, means other than said source ofalternating current for impressing a potential between ground and saidentire series circuit containing said cathode, anode, load deviceandsource, a detecting conductor connected to said control electrode andinsulated from ground, and variable impedance means connected directlybetween said control electrode and cathode for adjusting the potentialof said control electrode to a critical value with respect to saidcathode and anode such that a slight change in the potential of saidcontrol electrode produced by approach of a body to said detectingconductor renders said valve conductive.

6. A system as described in claim 5 in which said means for impressing apotential between ground and all parts of the said series circuit,supplies alternating current substantially in phase with the alternatingcurrent supplied by said source.

. 7. A system as described in claim 5 in which said means for impressinga. potential between ground and all parts of the series circuitincluding said source, said load device and said electrode is a sourceof alternating current and in which said variable impedance meanscomprises a variable condenser.

8. In a system for controlling an electric circuit in response to changeof a body with respect to some portion of said circuit; a source ofalternating current, a transformer having a primary winding and meansfor connecting said primary winding to said source, said transformerhaving a core of paramagnetic material and a secondary winding, meansfor maintaining one end of said secondary winding substantially atground potential, a load device, an electric valve having a cathode, ananode and a control grid, means connecting said cathode, anode and loaddevice in series with a portion of said secondary winding remote fromthe grounded end thereof, a detecting conductor connected to saidcontrol grid and insulated from ground, and variable impedance meansconnected directly between said grid and cathode for adjusting thepotential of said grid to a critical value with respect to said cathodeand anode such that a slight change in the potential of said gridproduced by approach of a body to said detecting conductor renders saidvalve conductive.

9. A system as described in claim 8 in which the said source ofalternating current is subject to potential fluctuations and in whichthe pri-. mary winding of said transformer has so few is cumulative, andmeans for varying the reluctance of the magnetic circuit of thewindings.

10. A system as described in claim 8 in which the secondary winding ofsaid transformer has a plurality of taps thereon adjacent the end remotefrom said grounded end, the number of turns between successive tapsbeing such as to provide substantially equal potentials between any twosuccessive taps thereon, whereby the same potential may be appliedbetween thecathode and anode of the valve but the valve as a wholemaintained at different potentials with respect to ground by connectingto different taps on said transformer.

11. A system as described in claim 5 in which a high resistance isconnected between said detecting conductor and said control grid circuitwhereby the current that can be drawn from said detecting conductor islimited to a value not dangerous to human life.

12. In a system for controlling an electric circuit in response tochange of a body with respect to some portion of said circuit, two setsof apparatus each comprising: a transformer having a primary winding anda secondary winding, one end of which is maintained substantially atground potential, a load device, an electric valve having a cathode,anode and control grid, means connecting said cathode, anode and loaddevice of each set in series with a portion of the secondary winding,remote from the grounded end thereof, of the transformer in that set,each set also comprising a detecting conductor connected to the controlgrid of that set and insulated from ground, the two detecting conductorsbeing spaced from each other across an area to be sensitized, each setcomprising variable impedance means connected between the grid andcathode of that set for adjusting the potential of the grid of each setto a critical value with respect to its associated cathode and anodesuch that a slight change in the potential of the grid renders the valveconductive, a single source of alternating current and means connectingthe primary windings of the transformers in said two sets of apparatusto said source with such polarity that the said two detecting conductorsare maintained at opposite potentials with respect to each other andwith respect to ground.-

13. In a system for controlling an electric circuit in response tochange of a body with respect to some portion of said circuit, two setsof apparatus each comprising, a transformer having a primary winding anda secondary winding, one end of which secondary winding is maintainedsubstantially at ground potential, aload device, an electric valvehaving a cathode, anode and control grid, means connecting said cathode,anode and load device ofeach set in series with a portion of thesecondary winding remote from the grounded end thereof of thetransformer in that set, each set also comprising a detecting conductorconnected to the control grid of that set and insulated from ground, thetwo detecting conductors being connected together, each set comprisingvariable impedance means connected between the grid and cathode of thatset for adjusting the potential of the grid of each set to a criticalvalue with respect to its associated cathode and anode such that aslight change in the potential of the grid renders the valve conductive,a single source of alternating. current and means connecting the primarywindings of the transformers in said two sets of apparatus to saidsource with such polarity that the said two detecting conductors aremaintained at substantially the same potentie whereby a slight change inthe potential of said grids produced by approach of a body to one ofsaid detecting conductors renders said valves conductive and whereby abreak in the connection between said two detecting conductors or a breakin any part of either one of said conductors upsets the balancedcondition of one of said valves suiliciently to initiate a dischargetherein.

14. A system as described in claim 1 in which said apparatus iscontained in a metal box connectedto said detecting conductor.

15. In a system for controlling an electric circuit in response tochange of a body with respect to some portion of said circuit, a sourceof alter nating current, a load device, an electric valve having acathode, anode and control grid, means connecting said cathode, anodeand load device in series with said source, means other than said sourcefor impressing a potential between ground and all parts of theseriescircuit including said source, said load device and said electrode,variable impedance means connected directly between said grid andcathode for adjusting the potential of said grid to a critical valuewith respect to said cathode and anode such that a slight change in thepotential of said grid renders said valve conductive, a detectingconductor connected to said grid, a second detecting conductor connectedto said anode, said first detecting conductor functioning in response toapproach of a foreign object thereto to change the potential of saidgrid sufilciently to render said valve conductive, and said seconddetecting conductor being relatively less sensitive than. said firstconductor but functioning in response to contact thereof by a foreignobject to render said valve conductive.

16. In a system for controlling an electric circuit in response tocontact of a foreign body with respect to some portion of said circuit,two sets of apparatus each comprising a transformer having a primarywinding and a secondary winding, one end of which secondary winding ismaintained substantially at ground potential, a load device, an electricvalve having a cathode and anode, means connecting said cathode, anodeand load device of each set in series with a portion of the secondarywinding of the transformer of that set, said portion being remote fromthe grounded end ofthe winding, each set also comprising a detectingconductor connected to the anode of that set and insulated from ground,the two detecting conductors being connected together, a single sourceof alternating current, and means connecting the primary windings of thetransformers in said two sets of apparatus to said source with suchpolarity that the said two detecting conductors are maintained atsubstantially the same potentials whereby an increase in the potentialof either anode produced by contact of a foreign body with one of saiddetecting conductors renders said valves conductive and whereby a breakin the connection between said two detecting conductors or a break inany part of either one of said conductors upsets the balanced conditionof one of said valves sufficient to initiate a discharge therein.

17 A system as described in claim 5, in which said apparatus iscontained in a metal box. said box being connected to said controlelectrode.

I GROVER C. HUBBLE.

